456 MONOCHROMATIC SUNLIGHT. 
473) he states that the ammonio-sulphate solution absorbs the red 
and yellow rays of the spectrum and with them so much of the — 
heat that but ‘twenty rays, for every hundred that fell upon it,” 
were transmitted. Ape 
In the London, Edinburgh, and Dublin Philosophical Magazine : 
for September, 1840 (Vol. xvii, p. 217) the same gentleman pub-- 
lished a paper “On the Process of Daguerreotype and its applica- 
tion to taking Portraits from the Life” in which he describes his 
attempts to reconcile the chemical and visual foci of portrait ob- A 
jectives, to-escape “the effulgence” of the solar rays thrown directly — , 
on the sitter, as practised at that time, “ abstract from them'theit 
heat and take away from them their offensive brilliancy.” These @ 
are almost the very objects for which microscopists to-day resort to , 
s 
the copper solution. Professor Draper employed in his experi- 
ments “a large trough of plate glass, the interstice being an inch ; 
thick” filled with a dilute solution of the ammonio-sulphate. Its 
size was about three feet square. This was so fixed in the couse 
of the sun’s rays, reflected from a mirror upon the sitter, that his 
head and the adjacent parts were illuminated only by the light — 
which had passed through the copper solution. By this device he 
reports he obtained excellent results. - 
In the spring of 1869 I received a letter from one of the nes 
of Professor Draper (dated April 19th) calling my attention — : 
above facts and transmitting several daguerreotypes of os 
copic objects all bearing the marks of considerable age. The 
writer (Prof. Henry Draper) states were made at various aue 
from 1851 to 1856. A Nachet microscope was used and in every 
case the ammonio-sulphate of copper is said to have been employed: 
The results are not particularly good as compared with penne 
photomicrographs, but appear to me not much inferior to ee ae 
that could have been done by the daguerreotype method wit w 
microscope used. The time was not yet ripe, and both microscop! < 
objectives and photographic methods have vastly improved 
_those days. 
My present purpose does not permit me to give 
to these reminiscences, the real object of this paper 
groate 
being " 
ically the advantages of monochromatic su 
definition. 
: ) use of the 
This object excludes a further consideration of the use’ 
